1. Field of the Invention
This invention relates generally to packaging of microwave circuits and more particularly concerns packaging that provides an ultra-low insertion loss feedthrough with particular interests in the high microwave frequencies and in applications using hermetically sealed packages for monolithic microwave integrated circuits.
2. Description of Related Art
Ultra high speed monolithic microwave integrated circuits (MMIC), microwave integrated circuits (MIC), other integrated circuits and hybrid circuit dies are mounted in environmentally protective or hermetically sealed packages that provide electromagnetic shielding and easy handling. Known manufacturing techniques include cofired ceramic enclosures using thick or thin film metallization, glass or quartz seals, ceramic enclosures using thin-film metallization, metal enclosures having ceramic feedthroughs, and metal enclosures having glass feedthroughs.
In cofired ceramic packages generally available for MMIC""s, the main contributor to poor microwave performance is the feedthrough. In conventional designs, discontinuity for poor performance exists due to the lead attachment, to passage of the conductor into and out from the ceramic wall, to changes in the conductor width and to coupling of RF signals to the lid and lid sealing ring. These discontinuities introduce higher-order modes and reflection as a result of impedance mismatch and contribute to overall poor feedthrough performance by having higher insertion loss. An MMIC package capable of good performance in the microwave range should have low insertion loss per feedthrough.
The insertion loss of a coaxial line or stripline formed on the feedthrough through a hermetically sealed ceramic wall increases with higher frequency, which results in a diminished signal strength. High insertion loss degrades MIC performance in many ways such as increased noise figure of small signal devices and reduced output power and efficiency of a power amplifier.
FIGS. 1-3 show a conventional feedthrough. The conventional feedthrough 200 has a substrate 210 (see FIG. 1) which is typically 15 mils thick and a block wall 220 mounted on the top surface of the substrate, as seen in FIGS. 1 and 2. The feedthrough 200 has a conductive microstrip line 240 traced on the top surface for transmitting RF signals. As seen in FIG. 1, the microstrip line 240 has three sections: an outer section 242, a middle 244 and an inner section 246. In general, the middle section 244 has a narrower width than that of the other sections.
As shown in FIG. 1, the block wall 220 covers substantially all of the middle section 244 of the microstrip line 240. The middle section 244 is formed on the substrate 210 using a tungsten layer which forms an hermetically sealed joint to the block wall 220. The outer and inner sections 242 and 246 are gold plated to at least 100 micro inches thick, but the middle section 244 is not and cannot be plated with gold, since it is already sealed with the block wall 220. As a result, the middle section 244 increases insertion loss. Moreover, the trace width of the middle section 244 is reduced to compensate for the additional parasitic capacitance due to the block wall 220, further adding to the series resistance for the middle section 244.
The bottom 249, (see FIG. 2), sides 247 and 247xe2x80x2 (see FIG. 1) and top 248 (see FIG. 2) surfaces of the feedthrough are also metallized and plated with nickel so that the feedthrough 200 can be brazed in place between the side wall and the base flange of the package making a hermetically sealed package.
At higher microwave frequencies, including millimeter wave, the thickness of the substrate 210 must be reduced to eliminate higher order transmission modes. The reduction of substrate thickness necessitates reduction in strip width of microstrip line 240, including all three sections 242, 244, and 246. This further increases the insertion loss due to the block wall 220 section by compensating for the parasitic capacitance arising from the block wall 220.
It is an object of the present invention to provide a high-frequency low-loss hermetic feedthrough which has an impedance compensated thin hermetic wall to provide high frequency packages and modules with low insertion loss.
It is another object of the present invention to provide an RF feedthrough suppressing surface modes.
Additional features and advantages of the invention will be set forth in the description which follows and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
According to one embodiment of the present invention, a feedthrough for an MMIC package has a substrate, a microstrip transmission line formed on the substrate for transmitting high frequency electronic signals and a wall disposed above the transmission line and the substrate. The wall of the feedthrough has varying thickness so that the narrowest width portion of the transmission line substantially crosses the narrowest portion of the wall. The narrowest portion of the wall may be created by placing two oppositely facing concave surfaces on each side of the wall. To reduce parasitic capacitance, the substrate and the wall may each have an air cavity imbedded in respective bodies near the vicinity of the transmission line.
According to one feature of the invention, the substrate of the feedthrough is made of dielectric ceramic material cofired to form an hermetic feedthrough.
According to other features of the invention, the transmission line has a first section, a second section and a middle section formed between the first and second sections. The middle section has a narrower width than that of the first and second sections. The first section of the transmission line extends to the narrower middle section by forming a transition edge of about a 90 degree angle. To minimize the area of the transmission line being crossed over by the wall, the narrow portion of the wall is preferably disposed above the middle section of the transmission line which also has the narrowest width. According to the present invention, the length of the middle section of the transmission line may be made longer or shorter than the width of the narrow portion of the wall, or none at all depending on the wall thickness of the narrowest width. This notch in the transmission line depends on the degree of compensation desired for increased parasitic capacitance due to the hermetic wall.
According to another embodiment of the present invention, a feedthrough for a MMIC package includes a substrate, a transmission line formed on the substrate for conducting an electronic signal and a wall having an inverse pyramid shape. In other words, the wall has a layered construction with a narrower lower portion and a wider upper portion. The narrower portion is preferably disposed substantially above and crosses the transmission line. The wall is made of a dielectric material and is disposed on the transmission line.
According to one feature of the invention, the substrate has an air cavity formed substantially above the transmission line. The wall comprises a plurality of layers constructed to have gradually increasing width from the narrower lower portion to the wider upper portion. These and other aspects, features and advantages of the present invention will be better understood by studying the detailed description in conjunction with the drawings and the accompanying claims.